1. Field of the Invention
The present invention relates to a damper disc assembly, and more particularly to a separate hub type damper disc assembly.
2. Description of the Related Art
A damper disc assembly is typically provided with an input member, an output hub, and a torsion spring disposed between the input member and the output hub in the circumferential direction, for transmitting a torque between the input member and the output hub. Torque is typically supplied by an engine connected to a portion of the damper disc assembly. The torque is then transmitted to the input member. Engagement between the input member and the torsion spring then causes the torque to be transmitted to the output hub.
The damper disc assembly serves to transmit the torque and to reduce vibrations generated during toque transmission. When vibration is transmitted to the input member, the input member is subjected to repeated relative rotation relative to the output hub. In this case, the torsion spring is repeatedly expands and is compressed to absorb the vibrations.
In one such damper disc assembly, there has been provided an output hub that is formed with two separate portions, specifically, a hub and a flange. The hub 102 shown in FIG. 5 is just such a hub. The flange 103, shown in phantom in FIG. 5, is a generally annular member disposed about the outer radial periphery of the hub. The flange is configured to undergo limited relative rotation with respect to the hub 102. A second spring member (not shown in FIG. 5) is disposed between the hub 102 and the flange 103 in a space or cutaway 102s. With such an arrangement, it is possible to dampen vibrations in wide frequency and intensity ranges. When the twist vibration is transmitted from the engine to the damper disc assembly, if the twist vibration has a small displacement angle, low rigidity characteristics of the second spring member cause it to undergo expansion/compression between the hub and the flange. If the displacement angle is large, the high rigidity torsion spring is also subjected to repeated expansion/compression.
The flange 103 is formed with a cutaway corresponding to the cutaway 102s shown in FIG. 5. The hub 102 is also formed with a plurality of engagement projections 102c on its outer circumferential portion. The flange 103 has, in the circumferential portion, corresponding engagement portions that are shaped to engage the engagement projections 102c of the hub 102 in response to relative rotation therebetween.
The engagement projections 102c of the hub repeatedly engage corresponding engagement cutaway portions of the flange. An impact force is repeatedly applied to the engagement projections 102c of the hub 102 when the hub 102 and the flange 103 are brought into contact with each other through the relative rotation therebetween.
The impact force is repeatedly applied to both circumferential sides of the engagement projections 102c. The cutaway 102s is formed by cutting the hub in the vicinity of adjacent engagement projections 102c. Therefore, portions 102p are formed on the hub 102 on either circumferential side of the cutaway 102s. It has been found that the rigidity of the hub 102 is weakened in the vicinity of the portions 102p. Often, the portions 102p and the immediately adjacent engagement projection 102c undergo plastic deformation and may be bent. Such deformation and bending can result in fatigue, reduced dampening characteristics and eventual failure of the hub 102. Fatigue caused by the repeated stress applied to the engagement projections 102c adjacent to the cutaway 102s may considerably affect the durability of the hub 102. If the force applied to the engagement projections 102c could be reduced, it would be possible to reduce the size of the engagement projections while keeping the sufficient durability (fatigue service life), so that the weight of the hub may be reduced.